Method of producing magnetic bubble domain devices

ABSTRACT

Surface layers of platelets of magnetic bubble domain material are polished while the platelet is subjected to a magnetic field. Considerably lower value magnetic fields are used, compared with conventional methods in which the platelets are exposed to high value magnetic field after polishing.

United States Patent [191 Lucas 5] A r. 10 1973 [54] METHOD OF PRODUCINGMAGNETIC [56] References Cited BUBBLE DOMAIN DEVICES UNITED STATESPATENTS [75] Inventor: John Martin Lucas, Ottawa, 1 Ontario Canada 12,618,108 11/1952 Richardson ..5l/216 R 2,053,475 9/1936 Gredys ..51/294[73] Assignee: Bell Canada-Northern Electric 1 Research Li Ottawa, O iPrimary Examiner-Donald G. Kelly Canada AttorneySidney T. Jelly [22]Filed: Sept. 30, 1971 57 ABSTRACT [21] PP N04 185,331 Surface layers ofplatelets of magnetic bubble domain material are polished while theplatelet is subjected to [521V U S C] 51/323 a magnetic field.Considerably lower value magnetic [5 I] In. .Cl 1/00 fields are used,Compared with conventional methods 58] d 317 318 in which the plateletsare exposed to high value magie 0 re 51/323, 324: netic field afterpolishing 6 Claims, 2 Drawing Figures 1 l0 I3 '15 A l/ a I? /I3 METHODOF PRODUCING MAGNETIC BUBBLE DOMAIN DEVICES The invention relates tomagnetic bubble domain devices, and in particular to the production ofmagnetization of desired orientation in surface layers of platelets ofmagnetic bubble domain material.

In the production of magnetic bubble domain devices, a wafer or plateletof suitable material, for example a rare earth orthoferrite, is polishedon both surfaces. In use a bias field is applied to enable stablemagnetic bubble domains, hereinafter referred to as magnetic bubbles, tobe generated and propagated in the platelet.

It is possible to produce self-biasing surface layers on such platelets,the device being of the form in which the platelets have a surfacecoercivity such that the magnetic bubbles are contained entirely in thecore layer not extending into the surface layers. By saturation of thesurface layers in a magnetic field of the correct polarity ofself-biasing magnetic field can be produced by means of the surfacelayers, the field providing some or all of the bias field necessary forstable magnetic bubbles.

To produce the magnetized layers it is necessary to apply a highmagnetic field, for example several kilo Oersteds. Another way is tocool the platelets from above the Curie temperature in somewhat lowervalue magnetic fields.

The present invention provides for the production of magnetized surfacelayers whereby considerably lower value magnetic fields are used, forexample of the order of I Oersteds. The magnetic field is maintainedwhile polishing the surface layers. By this means the surface layershave a domain configuration as required.

A further feature of the invention is that a platelet can have itssurface layers produced with the magnetization of the layers of oppositepolarity.

Thus in accordance with one feature of the invention a method ofproducing platelets of magnetic bubble material comprises the steps ofpositioning a platelet in a magnetic field of desired polarity, andpolishing a surface of the platelet while exposed to the magnetic field.

In accordance with a further feature of the invention both surfaces of aplatelet are polished while the platelet is exposed to the magneticfield.

In accordance with yet another feature of the invention the magneticfield during the polishing of one surface is of opposite direction tothe magnetic field during polishing of the other surface.

In accordance with yet another feature of the invention a magneticbubble domain device comprises a platelet of magnetic bubble domainmaterial having high coercivity surface layers, the layers having a magnetization of opposed direction.

The invention will be understood by the following description of certainembodiments, by way of example, in conjunction with the accompanyingdrawings which are diagrammatic cross-sections of two platelets,wherein:

FIG. 1 is of a platelet with surface layers having magnetization withthe same direction; and

FIG. 2 is of a platelet with surface layers having magnetization withopposed direction.

In a platelet of magnetic bubble material single wall domain patternsare formed, the pattern being variable by the application of an externalbiasing magnetic field. By application of a suitable field the domaincan be caused to become bubbles, that is cylindrical areas of aparticular magnetic polarity. This is well known.

To permit satisfactory propagation of magnetic bubbles it has hithertobeen necessary to prepare the platelets so that the surface layers havea very low coercivity -desirably less than 0.1 Oersted-- and that thesurface layers be free from any imperfections -such as scratchesas suchimperfections prevent satisfactory propagation of the magnetic bubbles.Even with carefully controlled polishing, handling of the polishedplatelets, or wiping with a cloth, can produce fine scratches which makethe platelet unsuitable.

To produce self-biasing surface layers the prepared platelets arepositioned in a high magnetic field usually of the value of severalkilo-Oerstedsthe field being normal to the platelet surfaces. The objectis to saturate the surface layers. Also cooling from above the Curietemperature in magnetic :field of a somewhat lower value can be used. Itis often difficult, or inconvenient, to provide the high value fieldsfor satisfactory saturation, and also forming and maintaining a magneticfield while cooling a platelet is inconvenient. Also, when prepared bysuch methods the bias field in each surface layer of a platelet is ofthe same polarity.

By the present invention the need for high value magnetic fields, or thecooling in. relatively high magnetic fields, is obviated. The plateletsare polished in a conventional manner, for example with chrome-oxide,while positioned in a magnetic field of relatively low value, forexample of the order of oersteds. The field applied is one whichproduces in the core layer the domain configuration which is required inthe surface layer. This configuration is required by the surface layerduring polishing. As previously stated, generally the desired domainconfiguration or pattern, corresponds to magnetization of the whole in asingle direction, that is as a single domain, but other configurationscan be produced by suitably adjusting the magnetic field. The domainconfiguration is first produced in the core layer, and while theconfiguration is maintained the surface layers are polished. Less than 1micron thickness need be removed in many cases. After removal of thislayer the new surface layer assumes the domain configuration originallyformed in the core layer. After removal from the magnetic field thesurface layers retain a magnetization which exerts a biasing effect onthe internal domain configuration in the core layer.

FIG. 1 illustrates diagrammatically to a greatly enlarged scale, aplatelet 10 having surface layers 11 and 12. The surface layers 11 and12 have been polished while the platelet was positioned in a magneticfield which was of a value sufficient to produce magnetic bubbles-indicated at 13. After the polishing was completed the platelet wasremoved from the magnetic field, the layers retaining a saturatedmagnetization as indicated at 14. The biasing effect of the domain wallsassociated with the layers, indicated at 15, was of a value sufficientto maintain the magnetic bubbles 13 in a stable condition. The magneticbubbles 13 are readily propagated by any normal method, such as by apermalloy drive pattern, or a conductor circuit on a surface of theplatelet. Usually the surfaces of the platelet are polishedindividually, but can be polished simultaneously if a suitable polishingdevice is available.

By polishing the surfaces individually, the surface layers can be givenmagnetizations of opposed direction. Such a platelet is illustrated inFIG. 2. The platelet 20 has surface layers 21 and 22 which were polishedsequentially while the platelet was positioned in a magnetic field. Themagnetic field was in one direction --normal to the platelet surfaceswhile one surface was polished, and the field was in the other directionwhile the other surface was polished. This is readily obtained merely byturning the platelet over while the magnetic field is maintained in thesame direction. As a result the magnetization of the surface layer 21 isin the opposite direction to the magnetization of the surface layer 22,as indicated at 23 and 24 respectively. With opposed surfacemagnetization the magnetic bubbles are not of cylindrical form but of aform which is circular in plan form -as viewed normal to the plateletsurfaceand of form somewhat humpshaped when viewed in cross-sectionparallel to the platelet surface, as indicated at 25.

It is possible, by means of the invention, to produce platelets withself-biasing surface layers, the self-biasing property being produced atintermediate stages during platelet polishing. The lower magnetic fieldsnecessary may be more readily applied during these stages. The highmagnetic fields normally required are not necessary and production iseasier and cheaper.

Platelets in accordance with the invention are easily manufactured, andretain the advantage that the surface finish does not have to be to thevery high standards required for conventional platelets.

The invention can be applied to all magnetic bubble domain materialwhich exhibit the surface layer effect, that is having somemagneto-strictive effect. This effect may be inherent in the material ormay be as a result of deliberately providing for impurities which willresult in a magneto-strictive effect.

What is claimed is:

1. A method of producing magnetic bubble domain devices, comprising thesteps of positioning a platelet of magnetic bubble domain material in amagnetic field of desired polarity normal to the platelet surfaces, andpolishing a surface of the platelet while the platelet is exposed to themagnetic field.

2. A method as claimed in claim 1, the magnetic field of a valuesufficient to produce a desired domain configuration in the core layerof the platelet prior to polishing said surface.

3. A method as claimed in claim 2, the magnetic field of a valuesufficient to produce a single domain in the core layer.

4. A method as claimed in claim 1, both surfaces of the platelet beingpolished while the platelet is exposed to the magnetic field, to producemagnetization of the surface layers of the same direction.

5. A method as claimed in claim 1, one surface of the platelet polishedwhile the platelet is exposed to a magnetic field of a first direction,the other surface of the platelet polished while the platelet is exposedto a magnetic field of a second direction opposite to the firstdirection, relative to said platelet whereby the surface layers aremagnetized in opposed directions.

6. A method as claimed in claim 2, the magnetic field of a value of theordgr of 109 Oprst eds.

1. A method of producing magnetic bubble domain devices, comprising thesteps of positioning a platelet of magnetic bubble domain material in amagnetic field of desired polarity normal to the platelet surfaces, andpolishing a surface of the platelet while the platelet is exposed to themagnetic field.
 2. A method as claimed in claim 1, the magnetic field ofa value sufficient to produce a desired domain configuration in the corelayer of the platelet prior to polishing said surface.
 3. A method asclaimed in claim 2, the magnetic field of a value sufficient to producea single domain in the core layer.
 4. A method as claimed in claim 1,both surfaces of the platelet being polished while the platelet isexposed to the magnetic field, to produce magnetization of the surfacelayers of the same direction.
 5. A method as claimed in claim 1, onesurface of the platelet polished while the platelet is exposed to amagnetic field of a first direction, the other surface of the plateletpolished while the platelet is exposed to a magnetic field of a seconddirection opposite to the first direction, relative to said plateletwhereby the surface layers are magnetized in opposed directions.
 6. Amethod as claimed in claim 2, the magnetic field of a value of the orderof 100 Oersteds.